ISS Propulsion Module

Critical ISS functionality such as guidance, navigation, control and propulsion are provided only by Russian (Zvezda and Progress) and the European (ATV) spacecraft.

The ISS requires an average 7,000 kg of propellant each year for altitude maintenance, debris avoidance and attitude control.

A Propulsion Module would have provided reserve propellant for one year of ISS orbit life in case of supply interruption.

Multiple supply vehicles are required to satisfy the ISS's 7,000 kg annual average propellant need.

Zarya FGB [Russian: ФГБ - Функциональный Грузовой Блок, English: Functional (or Operational) Cargo Block (or Module)] holds 5,500 kg and the Zvezda Service module holds 860 kg, however, it is generally preferred to keep the main thrusters on Zarya and Zvezda in reserve, as they have finite lifespans.